The invention relates to a bipolar electrode-electrolyte unit for electrochemical cells, like those used in double-layer capacitors, fuel cells, electrolyzers, gas sensors, or bifunctionally operated systems.
In a bipolar electrode, one side operates as a cathode and the other as an anode, and there is an electrically conducting connection impermeable to gases and electrolytes between the cathode and anode sides. Both sides are in contact with an ion conductor (electrolyte).
At the phase boundary between the electron and ion conductors, electrochemical processes occur that produce interface capacitances of several farads per cm.sup.3. Due to the nature of the electrochemical reactions and also as a result of the large porous surface, these electrodes are advantageously suitable for electrochemical energy converters and storage devices in which high currents flow because of a material reaction. In the simplest case, the current through the electrode cross section is described by Faraday's Law and the Butler-Volmer equation.
Electrochemical energy storage using bipolar electrodes was described in European Patent Document EP 0 622 515 B1. To achieve high operating voltages, an arbitrary number of bipolar electrodes are connected electrically in series and a so-called electrode stack is constructed.
An object of the present invention is to provide a bipolar electrode-electrolyte unit with improved conductivity, capacitance, electrochemical activity, and integration capability for electrochemical cells with a high energy and power density.
This and other objects and advantages are achieved by the electrode-electrolyte unit according to the invention, which comprises a bipolar electrode with a barrier layer as well as at least two carrier layers and at least two active layers. It has the following characteristics:
the active layers consist of an electroactive material with low resistance and high capacitance; PA1 the barrier layer is electrically conducting, impermeable to material, and consists of compressed and/or impregnated carbon, plastic bonded carbon, or carbon-coated sheet metal; PA1 the carrier layers located between the barrier layer and an active layer form an electrically conducting porous three-dimensional structure with a large specific and geometric surface that contains carbon fiber papers, carbon fiber films, carbon fiber mats, carbon fiber fabrics, compressed graphite flakes, and carbon fibers, conducting plastic or carbon produced by pyrolysis or dehydration of organic materials; PA1 the barrier layer and carrier layer form a stable mechanical and electrical bond produced by gluing, rolling, or compression; and PA1 the electrolyte is a good conductor.
By connecting several electrode-electrolyte units electrically in series (in a row or by stacking them on top of one another), an electrode stack is produced, with the electrolyte being located between two bipolar electrodes in each case.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.